It is often desirable to know information about liquid levels in reservoirs. Determining liquid levels in reservoirs, such as in sewage tanks, wells, water cisterns or tanks, and other liquid storage vessels, whether enclosed or open and exposed to the environment, has been done in a number of ways. For example, in tanks that are visually accessible, an operator may periodically take visual readings of the liquid level.
Visual readings, however, are often not desirable in systems where an automatic response is required when the liquid level reaches a certain threshold. In such cases the activation of a pump or valve may be necessary to move more liquid into the reservoir or to discharge liquid from the reservoir. In systems where visual readings are not available or when an immediate response is required, control systems are typically employed that are responsive to a liquid level indication. Such control systems may illuminate a light on an indicator panel representing the liquid level and/or trip an audible alarm to notify a human operator that corrective action is required.
Examples of liquid level sensing devices for use with wastewater reservoirs or other liquid holding vessels are discussed in U.S. Pat. No. 6,595,051 of Jul. 22, 2003 and U.S. Pat. No. 6,443,005 of Sep. 3, 2002 which are both incorporated by reference herein.
Different types or configurations of reservoirs often require different types or configurations for control systems associated with the reservoirs. For example, some reservoirs may have one pump while other reservoirs have more than one pump which are capable of moving liquids out of the reservoir. In addition, some reservoirs may include liquid level sensing devices in the form of mechanical floats positioned at various levels in the reservoir, while other reservoirs may use liquid level sensing devices such as a pressure bell located at the bottom of the tank, such as shown in U.S. Pat. No. 6,595,051.
Wastewater control systems are often designed to cause one or more pumps to start pumping liquid out of a reservoir responsive to the level of liquid in the reservoir. It is often desirable to use pumps with single phase motors for this purpose. Because single phase motors do not have multiple phases, to begin rotation, start windings and start capacitors (herein after referred to as the start circuit) are typically required to increase motor torque and achieve motor acceleration. Historically, this has been achieved with the use of a potential relay that energizes when the motor start winding generates enough voltage potential, which is based on motor rotational speed. In response to the potential relay energizing, the relay can be configured to drop out (e.g., remove power from) the start circuit. Because start capacitors operate in series with the start winding, the relay that controls power to the start circuit must open at the correct time so that the voltage does not build too high and blow up the start capacitor. Thus, to increase the reliability and safety of such control systems, there exists a need for a control system which can reliably prevent start capacitors from being damaged and causing damage.